This was best sent as a PM to a staffmember, due to it being for your own personal forum.
which is why I shall archive this thread in the next day or so.

Its not working due to your initialization of the backgrounds array, and/or likely the css layout of your forum which I do not have the ability to help you with. The element/object accesssible via #header is likely not the same.

The forum also uses 3rd party libraries I believe, hence the $ functions. I do not recall which library it is, but I assume AJAX.
Since you don't want the slideshow, you really just need to replace the default square theme header image with whatever image you want so this is extremely roundabout way of doing that.